Synthetic polymers are a broad family of materials with a remarkable range of applications. The fundamental building blocks for polymers are called monomers, and numerous methods have been devised for producing polymers from monomers. The earliest efforts to produce polymers focused on controlling the molecular formulas of polymers and producing useful materials from readily available chemical feedstocks. As the field advanced, the importance of molecular structure in dictating many polymer properties became apparent, and techniques for controlling the molecular structures of polymers began to emerge. Both the number of techniques and their ability to achieve structural control at the molecular level has increased greatly over the past two decades. This trend is expected to continue as most areas of science and engineering increasingly focus on controlling the structure and properties of materials on a size scale ranging from several nanometers to several hundred nanometers.
For synthetic polymers, molecular structures are most often controlled during the polymerization process, which typically involves the formation of macromolecules from many smaller molecules (e.g., monomers). The degree of control depends on many factors and there is considerable debate about the nomenclature to be used in describing various forms of “controlled polymerization”. However, there is a growing consensus that use of the term “controlled polymerization” is appropriate when describing processes from which polymers with predetermined molar masses and low polydispersities can be obtained. Polymerization also can be defined as “controlled” if side reactions occur, but only to an extent which does not considerably disturb the control of the molecular structure of the polymer chain. Most major classes of chain polymerization, including anionic, cationic, ring-opening metathesis (ROMP), coordination and radical polymerization, can be performed as “controlled” polymerization processes under appropriate conditions.
Regardless of the process, the key to achieving the conditions necessary for controlled polymerization is to facilitate productive steps in the process while discouraging unwanted side reactions. Historically, this has been accomplished in part by minimizing or eliminating water from the system. With the discovery of more functional group tolerant catalysts for coordination polymerization, olefin metathesis and cationic polymerization, as well as methods for achieving controlled free radical polymerization, the presence of water no longer represents an insurmountable obstacle. In fact, the continuing drive for more environmentally benign, water-based manufacturing processes and products provides strong incentives for developing aqueous processes for controlled polymerization. Controlled emulsion polymerization is particularly attractive for water-insoluble monomers and polymers, and there is intense worldwide competition in both academic and industrial circles to develop practical emulsion processes.
A review of the literature indicates that, in general, conventional emulsion polymerization techniques do not work well for controlled polymerization. In many cases, the fundamental problems are related to slow initiation coupled with slow transport of the “active” agent or its precursor through the water phase and into the growing polymer particles. In order to circumvent these problems, many groups have used newer techniques for achieving better emulsions and faster rates. The most common technique is “miniemulsion”. With this technique, a preformed conventional emulsion of monomer(s), surfactant, a hydrophobe and water is treated under high shear conditions with a homogenizer or ultrasonic horn to prepare much finer, self-stabilized droplets. The fine droplets become the locus for polymerization, bypassing the need for transport through the water phase. The two main drawbacks of the miniemulsion technique are: (1) the need for specialized and expensive equipment, and (2) the use of a hydrophobe (e.g., hexadecane), which is undesirable for many potential applications.
A second technique for producing fine droplets is “microemulsion”, which typically produces initial monomer droplets in the range of 5 nm and final polymer particles in the range of 30-40 nm. This technique usually requires very large amounts of surfactant, and it rarely is used for controlled polymerization because the amount of surfactant often equals or exceeds the amount of monomer present.
A third technique for achieving controlled emulsion polymerization utilizes a seeding process to initiate polymerization. With this technique, a fraction of the monomer is first mixed with initiator, control agent, water and surfactant. This combination is mixed and allowed to react for a period of time before additional monomer is added. The intent of the first stage is to allow the initiator to form “living” oligomers or “seeds” under conditions where the surfactant-to-monomer ratio is relatively large (i.e., microemulsion). Although this technique has some advantages over miniemulsion because it does not require a hydrophobe or specialized equipment, it does not solve the fundamental problems associated with the use of controlled polymerization technologies in emulsion, such as slow initiation or long reaction times compared to solution reactions.
In assessing this situation, what appears to be necessary for practical emulsion processes based on controlled polymerization technology is a method for: (1) producing stable emulsions without hydrophobes or special equipment; (2) utilizing conventional surfactants and soap levels; (3) effecting rapid initiation and propagation; and (4) achieving complete conversion within a reasonable period of time.